It is often necessary or desirable to monitor various parameters of blood and to obtain quantitative data concerning such parameters in real time. In order to accomplish this, blood is caused to flow through a flow-through housing past sensors which provide signals representative of the parameters of interest. For example, Cooper U.S. Pat. No. 4,640,820 shows a flowthrough housing with fluorescent sensors which respond to the partial pressure of oxygen, the partial pressure of carbon dioxide and the pH of blood which has passed through the flow-through housing.
Prior to using the flow-through housing, the sensors must be calibrated. One calibration technique, which is used for the flow-through housing and sensors shown in the Cooper patent is to immerse the flow-through housing in a calibration liquid and bubble the gas or gases of interest through the calibration liquid. A similar technique is utilized to calibrate the sensors shown in Maxwell Patent No. 4,830,013.
This calibration technique, which employs an essentially static calibration liquid, is most satisfactory when used in conjunction with a device having a flow-through passage of sufficient cross-sectional area so that the calibration liquid can readily fill all portions of the passage to fully expose the sensors to the gases carried by the calibration liquid. However, for some applications, it is desirable to utilize a flow-through housing having a relatively small cross-sectional area. In fact, the cross-sectional area is sufficiently small so that the surface tension may prevent the calibration liquid from completely filling the flow-through passage and adequately exposing the sensors to the gases in the calibration liquid. Another complicating factor is that the flow-through housing must subsequently be used for medical purposes.
These problems are solved as set forth in common assignee's application Ser. No. 07/514,704 entitled Sterile Loop Calibration System naming Thomas Maxwell and Thomas Hacker as coinventors and filed on even date herewith. According to the invention of this copending application, sterile calibration liquid is pumped through the flow-through passage.
A peristaltic pump can be used for this purpose because it will allow the calibration liquid to remain sterile. One prior art peristaltic pumping system includes a reusable component and a disposable component. The reusable component includes a motor and a rotatable cam, and the disposable component includes a cassette with a compressible tube through which the liquid to be pumped can flow. The disposable cassette can be loaded into the reusable component to enable the cam to progressively squeeze the tube to pump the liquid.
Although a system of this type does maintain sterility because the liquid being pumped is isolated from the cam, the cam is positioned to squeeze a region of the tube during storage, and this may cause the tube to take a permanent "set" and become occluded or partially occluded. In addition, it is sometimes relatively difficult to load the cassette into the reusable component because of the friction between the cam and the tube. In this regard, the tube is typically constructed of a soft, deformable material, such as silicone, and as such, the tube has a tacky or highfriction surface which inhibits sliding movement with the cam.
Horres et al U.S. Pat. No. 4,559,040 attempts to solve these problems by providing the disposable component with a movable cap so that, by appropriately angularly orienting the cam, the cam will not compress the tube during storage. Unfortunately, this construction requires precise assembly of the disposable component prior to use and precise angular indexing of the drive shaft relative to the cam in order to drivingly couple the drive shaft from the reusable component to the cam.